Nonvolatile semiconductor memory device, semiconductor device and manufacturing method of nonvolatile semiconductor memory device

ABSTRACT

A nonvolatile semiconductor memory device includes a gate portion formed by laminating a tunnel insulating film, floating gate electrode, inter-poly insulating film and control gate electrode on a semiconductor substrate, and source and drain regions formed on the substrate. The tunnel insulating film has a three-layered structure having a silicon nitride film sandwiched between silicon oxide films. The silicon nitride film is continuous in an in-plane direction and has 3-coordinate nitrogen bonds and at least one of second neighboring atoms of nitrogen is nitrogen.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/491,054,filed Jul. 24, 2006, which is based upon and claims the benefit ofpriority from prior Japanese Patent Applications No. 2005-214613, filedJul 25, 2005; and No. 2006-123914, filed Apr. 27, 2006, the entirecontents of each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a nonvolatile semiconductor memory device witha stacked gate structure having a floating gate electrode and controlgate electrode stacked on a semiconductor substrate and a manufacturingmethod thereof. Further, this invention relates to a semiconductordevice having a gate electrode formed on a semiconductor substrate witha gate insulating film disposed therebetween.

2. Description of the Related Art

Recently, miniaturization of elements is rapidly advanced in anelectrically programmable and erasable nonvolatile semiconductor memory(EEPROM). In the EEPROM, high voltage is applied to the control gateelectrode in the case of programming. Then, electrons are injected fromthe substrate side into the floating gate electrode which lies under aninter-poly insulating film via a tunnel insulating film. In the case oferasing, electrons in the floating gate are extracted.

For injection or extraction of electrons with respect to the floatinggate, high voltage is required and great stress is applied to the tunnelinsulating film formed of an oxide film. As a result, defects calledtraps are formed in the tunnel insulating film. Therefore, a leakcurrent increases and the data holding characteristic will bedeteriorated. The leak current caused by application of the stressdepends on the film thickness of the tunnel insulating film to a greatextent and the leak current more easily flow as the film thicknessbecomes smaller. The phenomenon becomes a main factor which prevents thetunnel insulating film from being made thin.

As the measure for solving the above problem, a method for reducing theleak current by introducing nitrogen into the tunnel insulating film toincrease the dielectric constant of the tunnel insulating film andincrease the physical film thickness is used. In this case, nitrogen isintroduced into the tunnel insulating film by annealing a silicon oxidefilm in ammonium (NH₃) gas, nitrogen monoxide (NO) gas or dinitrogenmonoxide (N₂O) gas. However, in the process using NH₃ gas, for example,an after-heat treatment at high temperatures is required to securelymaintain the high reliability since a large amount of hydrogen isintroduced in the tunnel insulating film. Further, in a process using NOor N₂O gas, there occurs a problem which is associated with thereliability and in which a hole trap amount at the stress applicationtime increases (JP-A H1-307272 (KOKAI)).

Further, recently, using an insulating film with a dielectric constantlarger than that of a conventional silicon oxide film or siliconoxide-nitride film as an inter-poly insulating film is studied withminiaturization of the nonvolatile semiconductor memory device. However,when an insulating film with the large dielectric constant such as ametal oxide film is sued as the inter-poly insulating film, metal isdiffused through the floating gate electrode and further diffused intothe tunnel insulating film, thereby causing the reliability of thetunnel insulating film to be deteriorated.

BRIEF SUMMARY OF THE INVENTION

An object of this invention is to form a tunnel insulating film of anonvolatile semiconductor memory device into an insulating film withhigh quality in which defects are difficult to occur and reduce a leakcurrent.

According to one aspect of this invention, there is provided anonvolatile semiconductor memory device comprising a first gateinsulating film selectively formed on a main surface of a semiconductorsubstrate of a first conductivity type, the first gate insulating filmincluding a three-layered structure in which a first insulating filmcontaining at least silicon and nitrogen is sandwiched between a secondinsulating film containing at least silicon and oxygen and a thirdinsulating film containing at least silicon and oxygen, the firstinsulating film being formed of a film which is continuous in anin-plane direction and having 3-coordinate nitrogen bonds and at leastone of second neighboring atoms of nitrogen being nitrogen; a floatinggate electrode formed on the first gate insulating film; a second gateinsulating film formed on the floating gate electrode; a control gateelectrode formed on the second gate insulating film; and source anddrain regions of a second conductivity type formed on the main surfaceof the substrate.

According to another aspect of this invention, there is provided asemiconductor device comprising a gate insulating film selectivelyformed on a main surface of a semiconductor substrate of a firstconductivity type, the gate insulating film including a three-layeredstructure in which a silicon nitride film is sandwiched between siliconoxide films, the silicon nitride film being a film which is continuousin an in-plane direction and having 3-coordinate nitrogen bonds and atleast one of second neighboring atoms of nitrogen being nitrogen; a gateelectrode formed on the gate insulating film; and source and drainregions of a second conductivity type formed on the main surface of thesubstrate.

According to still another aspect of this invention, there is provided anonvolatile semiconductor memory device comprising an element region ofa first conductivity type formed in a wall shape on a substrate; a firstgate insulating film formed to cover part of the element region, thefirst gate insulating film including a three-layered structure in whicha silicon nitride film is sandwiched between silicon oxide films, thesilicon nitride film being formed of a film which is continuous in anin-plane direction and having 3-coordinate nitrogen bonds and at leastone of second neighboring atoms of nitrogen being nitrogen; a floatinggate electrode formed on the first gate insulating film; a second gateinsulating film formed on the floating gate electrode; a control gateelectrode formed on the second gate insulating film; and source anddrain regions of a second conductivity type formed in the element regionbetween which a gate portion is arranged, the gate portion being formedof the floating gate electrode, control gate electrode and first andsecond gate insulating films.

According to another aspect of this invention, there is provided amanufacturing method of a nonvolatile semiconductor memory device,comprising forming a first gate insulating film by oxidizing a mainsurface of a semiconductor substrate of a first conductivity type toform a first silicon oxide film, depositing an amorphous silicon film onthe first silicon oxide film, nitriding the amorphous silicon film toform a silicon nitride film and forming a second silicon oxide film onthe silicon nitride film; forming a floating gate electrode on the firstgate insulating film; forming a second gate insulating film on thefloating gate electrode; forming a control gate electrode on the secondgate insulating film; forming a gate portion by etching a laminated filmof the floating gate electrode, the control gate electrode and the firstgate insulating film and the second gate insulating film into a gatepattern; and forming source and drain regions of a second conductivitytype in the substrate with the gate portion used as a mask.

According to another aspect of this invention, there is provided amanufacturing method of a nonvolatile semiconductor memory device,comprising forming a first gate insulating film by oxidizing a mainsurface of a semiconductor substrate of a first conductivity type toform a first silicon oxide film, depositing a crystalline silicon filmon the first silicon oxide film, nitriding the crystalline silicon filmto form a silicon nitride film and forming a second silicon oxide filmon the silicon nitride film; forming a floating gate electrode on thefirst gate insulating film; forming a second gate insulating film on thefloating gate electrode; forming a control gate electrode on the secondgate insulating film; forming a gate portion by etching a laminated filmof the floating gate electrode, the control gate electrode, the firstgate insulating film and the second gate insulating film into a gatepattern, and forming source and drain regions of a second conductivitytype in the substrate with the gate portion used as a mask.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross sectional view showing the schematic structure of anonvolatile semiconductor memory device according to a first embodimentof this invention.

FIG. 2 is a cross sectional view showing the concrete structure of atunnel insulating film used in the first embodiment.

FIG. 3 is a diagram showing the spectrum of nitrogen measured by aphotoelectron spectroscopy for the conventional structure and thepresent embodiment in which the bond state of nitrogen is a 3-coordinatebond.

FIGS. 4A to 4C are views showing the conventional manufacturing processof a tunnel insulating film and the bond state of nitrogen.

FIG. 5 is a characteristic diagram showing the relation between thedensity of nitrogen and bonding strength between silicon and nitrogen,for illustrating the first embodiment.

FIG. 6 is a characteristic diagram showing the relation between oxidefilm voltage and leak current density for tunnel insulating films in theconventional case and in the first embodiment.

FIGS. 7A and 7B are views showing MOS band structures, for illustratingthe effect of the first embodiment.

FIGS. 8A to 8D are cross sectional views of the steps showing thenecessity of continuity of the tunnel insulating film, for illustratingthe effect of the first embodiment.

FIG. 9 is a characteristic diagram showing the relation between thevoltage of the tunnel insulating film and capacity, for illustrating theeffect of the first embodiment.

FIG. 10 is a characteristic diagram showing the degree of roughness of asilicon nitride film, for illustrating the effect of the firstembodiment.

FIG. 11 is a diagram showing a desired range of nitriding temperatureand film-formation pressure when a silicon nitride film is formed bynitriding an amorphous silicon film, for illustrating the effect of thefirst embodiment.

FIGS. 12A to 12E are cross sectional views showing a manufacturingprocess of a nonvolatile semiconductor memory device according to asecond embodiment of this invention, and particularly, a manufacturingprocess of a tunnel insulating film.

FIG. 13 is a cross sectional view showing the schematic structure of aMOS transistor, for illustrating a modification of the first embodiment.

FIG. 14 is a diagram showing the result of analysis of oxygenconcentration in the silicon nitride film.

FIG. 15 is a cross sectional view showing the oxidizing state of thesilicon nitride film.

FIGS. 16A to 16I are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to athird embodiment of this invention.

FIG. 17 is a characteristic diagram showing the concentrationdistributions of nitrogen and oxygen in the tunnel insulating film, forillustrating the effect of the third embodiment.

FIGS. 18A to 18I are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to afourth embodiment of this invention.

FIGS. 19A to 19E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to afifth embodiment of this invention.

FIG. 20 is a cross sectional view showing the element structure of oneexample of the nonvolatile semiconductor memory device formed in thefifth embodiment.

FIG. 21 is a cross sectional view showing the schematic structure of anonvolatile semiconductor memory device according to a sixth embodimentof this invention.

FIG. 22 is a characteristic diagram showing the width of the depletionlayer in a floating gate electrode, for illustrating the effect of thesixth embodiment.

FIG. 23 is a perspective view showing the schematic structure of afin-type nonvolatile semiconductor memory device according to a seventhembodiment of this invention.

FIGS. 24A to 24H are cross sectional views showing the manufacturingprocess of the fin-type nonvolatile semiconductor memory device of theseventh embodiment.

FIGS. 25A to 25E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to aneighth embodiment of this invention.

FIGS. 26A to 26E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to aninth embodiment of this invention.

FIG. 27 is a diagram showing a variation in the film thickness ofnitride films formed when polysilicon films of different depositiontimes are subjected to a nitriding process in the same condition.

FIG. 28 is a characteristic diagram showing the difference in the SILCcharacteristic according to the formation condition of a nitride film.

FIG. 29 is a characteristic diagram showing the relation betweennitriding time and nitride film thickness.

FIG. 30 is a diagram showing a variation in the characteristic of anitride film according to a difference in the nitriding time.

FIG. 31 is a diagram showing the relation between deposition time ofpolysilicon and time required for completely nitriding the same.

FIG. 32 is a diagram showing a difference in incubation time ofpolysilicon according to a difference in the background.

FIG. 33 is a diagram showing the insulation characteristics of nitridefilms which are different depending on whether a polysilicon film beforenitriding is continuous or discontinuous.

FIG. 34 is a diagram showing a variation in the surface roughness whensilicon is deposited on the silicon oxide film and when silicon isdeposited after dangling bonds are formed in the surface of the siliconoxide film.

FIGS. 35A to 35E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to atenth embodiment of this invention.

FIG. 36 is an equivalent circuit diagram showing the circuitconfiguration of a CMOS semiconductor device according to an eleventhembodiment of this invention.

FIG. 37 is a cross sectional view showing the schematic elementstructure of the CMOS semiconductor device according to the eleventhembodiment.

FIG. 38 is a diagram showing the characteristic of a CMOS inverteraccording to the eleventh embodiment in comparison with that of theconventional CMOS inverter.

DETAILED DESCRIPTION OF THE INVENTION

There will now be described embodiments of this invention in detail withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a cross sectional view showing the schematic structure of anonvolatile semiconductor memory device according to a first embodimentof this invention.

A silicon thermal oxide film (element isolation insulating film) 12 forelement isolation is embedded in and formed on a p-type (firstconductivity type) silicon substrate (semiconductor substrate) 11 tosurround the element forming area of the main surface of the substrate11. On the main surface of the silicon substrate 11, a floating gateelectrode 14 is formed with a tunnel insulating film (first gateinsulating film) 13 disposed therebetween. On the floating gateelectrode 14, a control gate electrode 16 is formed with an inter-polyinsulating film (second gate insulating film) 15 disposed therebetween.

The inter-poly insulating film 15 is an ONO film with a thickness of 7nm formed of a silicon oxide film, CVD silicon nitride film and CVDsilicon oxide film, for example. The floating gate electrode 14 andcontrol gate electrode 16 are formed of a polysilicon film.

On the side walls of the gate electrode portion formed of the tunnelinsulating film 13, floating gate electrode 14, inter-poly insulatingfilm 15 and control gate electrode 16, a silicon oxide film 17 isformed. On the main surface of the substrate 11, n-type (secondconductivity type) source and drain diffusion layers 18 are formed byion-implanting phosphorous.

The basic configuration explained so far is the same as that of theconventional device, but in the present embodiment, the configuration ofthe tunnel insulating film 13 is different from that of the conventionaldevice. That is, as shown in FIG. 2, the tunnel insulating film 13 ofthe present embodiment is formed to have a laminated structure in whicha silicon nitride film (first insulating film) 13 c is sandwichedbetween a silicon oxide film (second insulating film) 13 b and a siliconoxide film (third insulating film) 13 c. In this case, the siliconnitride film 13 a can be obtained by depositing an amorphous siliconfilm on the silicon oxide film 13 b which is formed on the substrate 11and nitriding the amorphous silicon film.

By nitriding the amorphous silicon film deposited on the silicon oxidefilm 13 b, as shown in FIG. 3, the bond state of nitrogen can be setinto a 3-coordinate bond state (that is, a state in which three bonds ofnitrogen are connected to respective silicon atoms). Further, in FIG. 3,the spectrum of nitrogen measured by the photoelectron spectroscopy isshown. It is observed that the peak position is shifted towards the lowbound energy side and a high-energy component is reduced in the presentembodiment in comparison with the conventional method. This shows thatthe nitrogen bond state is more completely set into the 3-coordinatebond state. In the example of FIG. 3, the most desirable state in whichall of the second neighboring atoms of nitrogen are nitrogen is set, butit is sufficient if at least one of the second neighboring atoms ofnitrogen is nitrogen.

The conventional silicon nitride film is formed as follows. That is,after a silicon oxide film 13 b is formed by oxidizing the siliconsubstrate 11 as shown in FIG. 4A, nitrogen atoms are introduced into thefilm 13 b by exposing the same to ammonium (NH₃) gas atmosphere as shownin FIG. 4B, for example. As a result, a silicon nitride film 13 a isformed as shown in FIG. 4C.

Nitrogen thus introduced into the film must break the bonds betweensilicon and oxygen whose bonding strength is high in the silicon oxidefilm 13 b and oxygen whose bond is broken at this time cannot beoutwardly diffused. Therefore, the complete 3-coordinate nitrogen bondcannot be formed. Then, an incomplete nitrogen bond state such as a2-coordinate nitrogen bond is formed.

For example, the 2-coordinate nitrogen bond is attained bydouble-bonding one bond of nitrogen with silicon and a non-bonding bondis not apparently provided. However, the bond is unstable and easilytends to trap a hole. This is caused by forming a level near the valenceband of silicon (silicon substrate) with which the silicon nitride filmmakes contact. In this case, the silicon nitride film captures holescoming from the silicon substrate side by voltage application and breaksone bond of nitrogen which is double-bonded with silicon. As a result,positive fixed charges tend to occur on the silicon side and non-bondedbonds and defects tend to occur on the nitrogen side.

On the other hand, in the tunnel insulating film 13 according to thepresent embodiment, nitrogen tends to have 3-coordinate nitrogen bondssince the silicon nitride film 13 a is directly formed. Further,according to the present embodiment, the density of 3-coordinatenitrogen bonds can be enhanced, nitrogen bonds are stable, andtherefore, nitrogen bond will not be easily broken by application ofelectrical stress.

FIG. 5 is a diagram showing enhancement of the bonding energy byenhancing the concentration of nitrogen based on the first principlecalculation. As is clearly understood from FIG. 5, elimination energyfor each atom becomes lower as the density of nitrogen atoms in siliconbecomes higher. That is, strong bonds corresponding to silicon-oxygenbonds of the silicon oxide film can be attained by aggregating nitrogenatoms at high concentration.

The electrical characteristic of a MOS capacitor formed by the abovemethod is shown in FIG. 6. FIG. 6 is a characteristic diagram showingthe tunnel insulating film of the present embodiment in comparison withthat of the conventional case. Leak currents (Stress Induced LeakageCurrents: SILC) which are increased by applying electrical stresses tothe respective tunneling insulating films are compared with each other.The stress induced leakage current is a leak current flowing through thedefects formed in the insulating film by application of electricalstress and the leak current increases as a large amount of defects areformed in the film.

According to FIG. 6, it is understood that the stress induced leakagecurrent is markedly increased by application of stress in the tunnelinsulating film of the conventional case, but it is not substantiallyincreased in the present embodiment. That is, it is understood thatformation of defects in the film is suppressed in the present embodimentand the tunnel insulating film in the present embodiment has highquality and is highly reliable. Further, in the present embodiment, thesilicon nitride film 13 a with 3-coordinate bonds which are stable instructure is formed in the central portion of the tunnel insulating film13 and sandwiched between the silicon oxide films 13 b and 13 c.Therefore, formation of defects near the central portion of the filmwhich most contributes to generation of the stress induced leakagecurrent can be suppressed.

Further, as shown in FIGS. 7A and 7B, the effect for the operation oflowering the write voltage is also attained. FIGS. 7A and 7B areschematic views showing the band structures of a MOS capacitor obtainedin this embodiment. The tunnel insulating film 13 in this embodiment hasa structure in which the silicon nitride film 13 a is sandwiched betweenthe silicon oxide films 13 b and 13 c. At the data holding time, thatis, in a state in which a weak electric field is applied to the tunnelinsulating film 13, as shown in FIG. 7A, the tunneling probability ofelectrons is small and no current flows due to the total film thicknessof barrier heights of the silicon oxide films 13 b, 13 c.

On the other hand, when voltage is applied to the above structure, asshown in FIG. 7B, a stronger electric field is applied to the siliconoxide films 13 b, 13 c with small dielectric constants. Then, electronstunnel through the thin silicon oxide films 13 b, 13 c and get over thesmall barrier height of the silicon nitride film 13 a so as to easilypass toward the anode side. As a result, both of the excellent dataholding characteristic and the operation of lowering the write voltagecan be attained. Further, although not shown here, the structure iseffective in suppressing deterioration of the reliability and interfacecharacteristic due to introduction of nitrogen which is well known inthe art since the silicon oxide film 13 b is formed on the interface onthe silicon substrate side.

FIGS. 8A to 8D are views for illustrating the effect of this embodiment.If the nitriding process of a silicon layer is performed at hightemperatures of, for example, 800° C. or more under a pressure of 100Torr (13300 Pa) or more, silicon nitride films 13 a cannot be formed asa continuous film in the in-plane direction as shown in FIG. 8A. Forexample, if an oxidation process is performed by using an oxidizingagent 20 such as oxygen for the above film, oxygen penetrates into gapsbetween the silicon nitride films 13 a as shown in FIG. 8B to oxidizecorresponding portions. As a result, a silicon oxide film 13 b which isnon-uniform in the in-plane direction is formed on the interface.

If the nitriding process is performed at temperatures of lower than 800°C. as in the present embodiment, for example, a silicon nitride film 13a which is continuous in the in-plane direction is formed as shown inFIG. 8C even under high pressure exceeding 100 Torr. If the above filmis oxidized by use of an oxidizing agent 20 such as oxygen, a siliconoxide film 13 c or an oxynitride film (silicon oxide-nitride film)containing nitrogen is uniformly formed only on the surface of thesilicon nitride film 13 a as shown in FIG. 8D. The silicon nitride filmwhich can be realized in the present embodiment can be formed as a filmwhich contains 3-coordinate nitrogen bonds with high density as shown inFIG. 3. Therefore, as shown in FIG. 5, the bonding strength of thenitrogen-silicon bond can be set as high as the bonding strength of theoxygen-silicon bond.

FIG. 9 is a characteristic diagram of the MOS structure including a gateinsulating film having a silicon nitride film sandwiched between siliconoxide films and shows cases where the silicon nitride film is formed asa continuous film and a non-continuous film. As known examples of thestructures having the continuous and non-continuous silicon nitridefilms each sandwiched between the silicon oxide films, a MONOS memory ordot memory is provided. In the above cases, the silicon nitride filmitself is used as a location of trapping charges to attain the memorycharacteristic. As an example, the present embodiment is compared withthe dots of the silicon nitride film (granular silicon nitride film).

Since the MONOS memory or dot memory utilizes generation of traps in thesilicon nitride film, the silicon nitride film used has 2-coordinatenitrogen bonds. Therefore, by applying a strong electric field, chargesare trapped in the silicon nitride film and the voltage-capacitancecharacteristic is shifted according to the polarity and an amount oftrapped charges. An example indicated by broken lines in FIG. 9indicates a case where electrons are trapped in the dots of the siliconnitride film. In the present embodiment, since a silicon nitride filmhaving 3-coordinate nitrogen bonds in which generation of traps issuppressed is used, defects in which electrons or positive holes can becaptured are difficult to be formed. Therefore, shifting of thevoltage-capacitance characteristic is not observed even afterapplication of strong electric field stress.

FIG. 10 is a characteristic diagram showing the film-formationtemperature, film-formation pressure and influences thereof exerted onthe film quality when a silicon nitride film is formed. If the nitridingprocess is performed at high temperatures of 800° C. or more underpressure exceeding 100 Torr (13300 Pa), an aggregated and non-continuoussilicon nitride film is formed in some cases. On the other hand, if thenitriding process is performed in a condition of temperatures of 800° C.or more and pressures of 100 Torr or less or in a condition oftemperatures lower than 800° C. and high or low pressure, a flat andcontinuous silicon nitride film can be formed. In the tunnel insulatingfilm forming process used in this embodiment, it is necessary to performthe process of forming the silicon nitride film at temperatures lowerthan 800° C. or at temperatures of 800° C. or more under pressures of100 Torr or less.

Further, the film-formation temperature and film-formation pressure atthe time of formation of the silicon nitride film are explained. FIG. 11is a diagram showing the result obtained in a case where the filmthickness and the degree of flatness of the silicon nitride film areexpressed by use of the nitriding temperature and film-formationpressure when an amorphous silicon film is nitrided.

As shown in FIG. 11, the film thickness and the degree of flatness ofthe silicon nitride film can be controlled according to the nitridingtemperature and film-formation pressure. In this invention, a flatsilicon nitride film having 3-coordinate nitrogen bonds at high densityis required. Therefore, a flat silicon nitride film can be obtained attemperatures lower than 600° C. as shown in FIG. 11, but the density of3-coordinate nitrogen bonds is low and it contains incomplete bonds suchas 2-coordinate nitrogen bonds. Therefore, generation of defects in thefilm which act as the source of a stress leak current cannot besuppressed. If the nitriding process is performed at high temperaturesof 800° C. or more under pressure higher than 100 Torr, a siliconnitride film having 3-coordinate nitrogen bonds at high density can beformed, but aggregation occurs in the course of formation of the siliconnitride film, the film becomes a granular film and the degree offlatness thereof is lowered. Therefore, it is desirable to nitrideamorphous silicon in a condition that the nitriding temperature is setin a range of 600° C. to 1000° C. and the nitriding pressure is set in arange of 10⁻¹ Torr and 100 Torr.

Thus, in the nonvolatile semiconductor memory device of the presentembodiment, the electric field on the silicon oxide film side can beenhanced by sandwiching the silicon nitride film 13 a having thedielectric constant which is twice larger than that of the silicon oxidefilm between the silicon oxide films 13 b and 13 c and the injectionefficiency is enhanced as shown in FIG. 7. Since the silicon nitridefilm 13 a having 3-coordinate nitrogen bonds aggregated at high densityis provided, generation of defects formed, particularly, by applicationof stress (that is, at the write or erase time in the strong electricfield) is suppressed and a stress induced leakage current is suppressed.Further, since the laminated structure of the silicon oxide films 13 b,13 c and the silicon nitride film 13 a is provided, defects caused byapplication of high voltage stress are suppressed from coupling thesubstrate side to the floating gate and dielectric breakdown becomesdifficult to occur. As a result, the service time of the memory devicebecomes long and the reliability thereof is made high.

Second Embodiment

FIGS. 12A to 12E are cross sectional views showing a manufacturingprocess of a nonvolatile semiconductor memory device according to asecond embodiment of this invention, and particularly, a manufacturingprocess of a tunnel insulating film. Portions which are the same asthose of FIG. 1 are denoted by the same reference symbols and the detailexplanation thereof is omitted.

First, as shown in FIG. 12A, the surface of a silicon substrate 11 isoxidized by use of oxygen gas, for example, to form a silicon oxide film13 b with a thickness of approximately 2 nm. Then, as shown in FIG. 12B,for example, a flat amorphous silicon film 13 d with a thickness ofapproximately 1 nm is formed on the silicon oxide film 13 b by use ofdisilane gas.

Next, as shown in FIG. 12C, the amorphous silicon film 13 d is directlynitrided at a temperature of 700° C. under a pressure of 50 Torr (6650Pa) by using a plasma nitriding method, for example. Thus, a siliconnitride film 13 a with a thickness of approximately 2 nm is formed. Atthis time, for example, nitrogen is used as a gas source for the plasmanitriding process, but if the nitriding process is performed by use of amixture of oxygen and nitrogen, a silicon nitride film (so-calledoxynitride film) having oxygen introduced therein can be formed.Further, the concentration of oxygen in the silicon nitride film can becontrolled to a desired concentration by changing the mixture ratio ofthe nitriding gas to oxidizing gas.

After this, the surface of the silicon nitride film 13 a is oxidized byuse of oxygen gas (oxidizing agent) 20, for example, to form a siliconoxide film 13 c with a thickness of approximately 2 nm. Thus, as shownin FIG. 12D, a tunnel insulating film 13 formed of the silicon nitridefilm 13 a sandwiched between the silicon oxide films 13 b and 13 c isformed. Then, as shown in FIG. 12E, an n-type polysilicon film with athickness of 200 nm having phosphorus doped therein is deposited as afloating gate electrode 14 on the tunnel insulating film 13. Thus, a MOSstructure can be formed.

After this, a nonvolatile semiconductor memory device can bemanufactured by depositing an inter-poly insulating film and apolysilicon film as a control gate and processing the above films intogate patterns.

Further, a normal MOS transistor can be formed as shown in FIG. 13 byprocessing the gate pattern in the state shown in FIG. 12E and formingsource and drain regions without depositing an inter-poly insulatingfilm and a polysilicon film. In FIG. 13, a reference symbols 53 denotesa gate insulating film formed by sandwiching the silicon nitride film 13a between the silicon oxide films 13 b and 13 c and a reference symbol54 denotes a gate electrode formed of polysilicon or the like.

FIG. 14 shows the result of analysis of oxygen concentration in thesilicon nitride film. When the nitriding process is performed by use ofonly nitrogen gas, a small amount of oxygen is introduced by aninfluence of residual oxygen. Further, if oxygen gas is added by anamount corresponding to 20% of nitrogen gas, oxygen of approximately 40%in the relative concentration with respect to nitrogen can be introducedinto the silicon nitride film.

As described above, if oxygen is contained in the silicon nitride film13 a, incomplete bonding portions of the silicon nitride film 13 a canbe replaced by oxygen and a film structure in which defects aredifficult to occur can be obtained. As another advantage attained byintroducing oxygen, a bandgap of the silicon nitride film can be widenedby introducing oxygen and a leak current can be reduced in the operationof the nonvolatile memory.

A case wherein oxygen is used as oxidizing gas added at the time offormation of the silicon nitride film is explained above, but thisinvention is not limited to the above case. For example, the same effectcan be attained by using oxidizing gas such as N₂O or NO. Further, amethod for introducing oxygen into the silicon nitride film by addingoxidizing gas at the nitriding time is explained, but this invention isnot limited to this case. Specifically, for example, an oxygen-addedamorphous silicon film is formed by adding oxygen gas to disilane gas atthe time of formation of an amorphous silicon film. An oxygen containingsilicon nitride film can be formed by use of a method for nitriding theoxygen-added amorphous silicon film in nitrogen plasma, for example.

Further, as another method, there is provided a method for forming thesilicon nitride film 13 a on the silicon substrate 11 by a directnitriding process and oxidizing the above film. If an extremely thinsilicon nitride film for logic is oxidized, oxygen can reach the siliconsubstrate (for example, JP-A 2001-190805 (KOKAI)).

However, the tunnel insulating film is thick and oxygen is difficult topass through a silicon nitride film having 3-coordinate nitrogen bondsat high density. Therefore, as shown in FIG. 15, only the surface of thesilicon nitride film 13 a is oxidized and oxygen is not introduced intothe interface between the silicon substrate 11 and the silicon nitridefilm 13 a. In the case of the tunnel insulating film structure, theelectron injection efficiency varies depending on the polarity ofvoltage applied and the write voltage and erase voltage in the flashmemory will vary. Therefore, it is desirable to provide the structure inwhich the silicon oxide films 13 b, 13 c are located in both of theinterfaces as in the present embodiment.

Although not shown here, if a plasma nitriding process is performed atthe room temperature, for example, in the nitriding process afterformation of the amorphous silicon film, an incomplete silicon nitridefilm having 3-coordinate nitrogen bonds at low density is formed unlikethe case of the nitriding process at high temperatures. In this case,the film contains defects or structures which are easily converted intodefects by application of electrical stress and does not satisfy thefunction as the tunnel insulating film.

However, for example, if the oxidizing process is performed bycombustion oxidization by use of oxygen and hydrogen at 750° C., asubstance containing oxygen or hydrogen and oxygen is diffused into thesilicon nitride film and defective sites in the silicon nitride film areterminated at oxygen or hydrogen to form an oxynitride film. In the thusobtained oxynitride film, the nitrogen concentration in the film can beenhanced in comparison with a method of introducing nitrogen into thesilicon oxide film which is a conventional method. Further, a weak bondin the silicon nitride film can be converted into an oxygen-silicon bondof high bonding energy and the film can be formed as a film in which astress induced leakage current is reduced and the reliability is madehigh.

In the present embodiment, the amorphous silicon film is nitrided, butthis is because it is desirable to form an amorphous silicon thin filmin order to make the tunnel insulating film flat. Further, when nitrogenatoms or nitrogen compounds tend to diffuse in the film and bondedtogether to form a nitride film structure at the nitriding time, it isdesirable to use a thin amorphous silicon film rather than a crystallinesilicon film in order to further enhance the nitriding efficiency andrealize a nitride film of high quality.

Further, the nitriding temperature is preferably set lower than thetemperature at which amorphous silicon is crystallized and the nitridingtemperature is preferably set lower than 700° C. This is because thedegree of roughness is enhanced by crystallization although thenitriding process can be performed even after the film is crystallized.Particularly, a silicon film is aggregated and set into a granular stateunder vacuum of high degree before the nitriding process in some cases.Therefore, it is desirable to perform the nitriding process attemperatures lower than temperatures at which the silicon layer iscrystallized in order to maintain the degree of flatness and nitridingefficiency of an amorphous film and form a tunnel insulating film withhigh quality and high reliability.

Third Embodiment

FIGS. 16A to 16I are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to athird embodiment of this invention. Portions which are the same as thoseof FIG. 1 are denoted by the same reference symbols and the detailexplanation thereof is omitted.

First, as shown in FIG. 16A, for example, a p-type (first conductivitytype) silicon substrate 11 having plane orientation (100) andresistivity 10 to 20 Ωcm is prepared and a groove is formed in thesurface of the silicon substrate 11. An element isolation insulatingfilm 12 with a thickness of approximately 0.6 μm is formed by burying aCVD oxide film in the groove. Then, an oxidizing process is performed at950° C. by use of oxygen gas to form a silicon oxide film 13 b with athickness of approximately 2 nm on the surface of the silicon substrate11. Next, for example, an amorphous silicon film 13 d of approximately1.5 nm is deposited at 400° C. under 1 Torr (133 Pa) by use of a thermalCVD method using disilane gas, for example.

After this, as shown in FIG. 16B, the amorphous silicon film 13 d isnitrided at the temperature of 700° C. under pressure of 50 Torr by useof a plasma nitriding method, for example. Thus, a silicon nitride film13 a with a thickness of 2 nm is formed. Next, as shown in FIG. 16C,oxygen atoms are introduced into the silicon nitride film 13 a byexposing the same to an oxygen gas atmosphere, for example, to form asilicon oxide film 13 c. As a result, as shown in FIG. 16D, a tunnelinsulating film (firs gate insulating film) 13 formed of the siliconnitride film 13 a sandwiched between the silicon oxide films 13 b and 13c is formed. In this case, as the silicon nitride film 13 c, a CVDsilicon oxide film deposited at 850° C. by use of dichlorosilane gas andoxygen gas, for example, can be used.

Next, as shown in FIG. 16E, an n-type polysilicon film with a thicknessof 200 nm having phosphorus doped therein is deposited as a floatinggate electrode 14 on the tunnel insulating film 13. Then, as shown inFIG. 16F, an ONO film with a thickness of 7 nm, for example, is formedas an inter-poly insulating film (second gate insulating film) 15 on thepolysilicon film. Then, as shown in FIG. 16G, an n-type polysilicon filmwith a thickness of 200 nm having phosphorus doped therein is depositedas a control gate on the ONO film 15.

Next, as shown in FIG. 16H, the resultant structure is patterned by useof a resist mask 21 and then the polysilicon film used as the controlgate electrode 16, the polysilicon film used as the floating gateelectrode 14, tunnel insulating film 13 and ONO film 15 are etched byuse of the reactive ion etching method to form a gate portion. Further,after the resist mask 21 is removed, the heat treatment is performed inthe oxidation atmosphere to restore the condition damaged by theprocessing and then a post-oxidation film 17 with approximately 3 nm isformed.

An unwanted portion of the floating gate electrode 14 in the gate widthdirection is removed by an RIE process or the like before formation ofthe control gate electrode 16. Then, an unwanted portion of the floatinggate electrode 14 in the gate lengthwise direction is removed at thesame time as the gate processing time of the control gate electrode 16.

Next, for example, phosphorus is ion-implanted into the entire surfacewith the concentration of 3×10¹⁵ cm⁻². After this, the heat treatment isperformed at 1000° C. for 20 seconds, for example, to diffuse phosphorusinto the silicon substrate 11 and activate the same. Thus, diffusionlayers 18 used as source and drain regions are formed to configure astructure as shown in FIG. 16I.

The succeeding process is not shown in the drawing here, but forexample, a silicon oxide film (inter-level insulating film) with athickness of 300 nm is deposited on the entire surface by the CVD methodand then contact holes are formed in the inter-level insulating film byan anisotropic etching process. After this, an aluminum film with athickness of 800 nm containing silicon and copper of 0.5%, for example,is formed and then patterned to form electrodes. Next, the heattreatment is performed at 450° C. for 15 minutes in a nitrogenatmosphere containing hydrogen of 10%.

Thus, according to the present embodiment, a nonvolatile semiconductormemory having the tunnel insulating film 13 of high quality and highreliability can be formed. Further, a leak current can be suppressed byconfiguring the tunnel insulating film 13 with the structure having thesilicon nitride film 13 a sandwiched between the silicon oxide films 13b and 13 c. If the thickness of the silicon nitride film 13 a is set to2 nm and the thickness of the silicon oxide films 13 b, 13 c is set to 2nm, the physical film thickness of the tunnel insulating film 13 is setto 6 nm and the tunnel insulating film 13 itself can be made thin. As aresult, the power supply voltage can be lowered and it becomes possibleto enhance the characteristic of the device and enhance the reliabilitythereof.

As the method for forming the silicon nitride film having the3-coordinate nitrogen bonds, a method in which the directly nitridingprocess using nitrogen plasma is taken as an example is explained, butthis is not limitative. The same effect can be attained in a nitridingprocess using ammonium (NH₃) gas and nitrogen radical. Since hydrogen isintroduced into the silicon nitride film if ammonium gas is used,hydrogen introduced into the film can be removed by performing the heattreatment at temperatures higher than the silicon nitride film formationtemperature under vacuum or in nitrogen or inert gas after formation ofthe silicon nitride film.

When the heat treatment is performed at temperatures higher than thenitriding temperature under vacuum or in nitrogen or inert gas afterformation of the silicon nitride film not only in a case of formation ofthe silicon nitride film by use of ammonium gas but also in a case offormation of a silicon nitride film by use of a different method, thecondition on the structure of the silicon nitride film can be furtheralleviated and a 3-coordinate silicon nitride film which is more stablein structure can be realized. Further, anti-oxidation is enhanced andoxidation is limited to the surface of the silicon nitride film bysubjecting the resultant structure to a high-temperature annealingprocess after nitriding.

In order to explain this state, the characteristic diagram is shown inFIG. 17. FIG. 17 indicates the result of measurement of concentrationdistributions of oxygen and nitrogen in the tunnel insulating film byuse of a secondary ion mass spectrometry. According to FIG. 17, asilicon oxide film or oxynitride film is formed on the surface of thesilicon nitride film, but diffusion of oxygen into the silicon nitridefilm is suppressed. However, the peak of oxygen concentration observednear the interface with the silicon substrate reflects the silicon oxidefilm before formation of the amorphous silicon film.

In the present embodiment, a case wherein the oxidation process usingoxygen (O₂) gas is performed in the oxidation process (FIG. 16C) afterformation of the silicon nitride film is explained as an example, butthis is not limitative. For example, the same effect can be obtained byperforming the oxidation process using ozone (O₃) gas, gas containingsteam (H₂O), oxygen radical or monoxide dinitride (N₂O) gas. The siliconoxide film 13 c on the silicon nitride film can be converted into anoxynitride film with desired nitrogen concentration by use of acombination of the above gases.

Fourth Embodiment

FIGS. 18A to 18I are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to afourth embodiment of this invention. Portions which are the same asthose of FIG. 1 are denoted by the same reference symbols and the detailexplanation thereof is omitted.

As shown in FIGS. 18A to 18E, a groove is formed in the surface of ap-type silicon substrate 11 having plane orientation (100) andresistivity 10 to 20 Ωcm by use of the same process as that shown inFIGS. 16A to 16E and an element isolation insulating film 12 with athickness of approximately 0.6 μm is buried and formed. Then, a siliconoxide film 13 b of approximately 2 nm is formed on the surface of thesilicon substrate and an amorphous silicon film 13 d is deposited. Next,after a silicon nitride film 13 a with a thickness of 2 nm is formed byusing a plasma nitriding method, a silicon oxide film 13 c is formed byintroducing oxygen atoms 20. Thus, the tunnel insulating film 13 isformed of the silicon nitride film 13 a sandwiched between the siliconoxide films 13 b and 13 c. After this, an n-type polysilicon film with athickness of 200 nm having phosphorus doped therein is deposited as afloating gate electrode 14 on the tunnel oxide film 13.

Next, as shown in FIG. 18F, a lanthanum aluminate film (LaAlOx) 15 bwith a thickness of 15 nm, for example, is deposited on the polysiliconfilm 14 by the CVD method. Then, as shown in FIG. 18G, an n-typepolysilicon film with a thickness of 200 nm having phosphorus dopedtherein is deposited as a control gate electrode 16 on the lanthanumaluminate film 15 b.

After this, as shown in FIG. 18H, the resultant structure is patternedby use of a resist mask 21 and then the polysilicon film used as thecontrol gate electrode 16, the polysilicon film used as the floatinggate electrode 14, tunnel insulating film 13 and lanthanum aluminatefilm 15 b are etched by the reactive ion etching method to form a gateportion. Further, after the resist mask 21 is removed, the heattreatment is performed in the oxidation atmosphere to restore thecondition damaged by the processing and a post-oxidation film 17 withapproximately 3 nm is formed. Next, for example, phosphorus ision-implanted into the entire surface with the concentration of 3×10¹⁵cm⁻². After this, the heat treatment is performed at 1000° C. for 20seconds, for example, to diffuse phosphorus into the silicon substrate11 and activate the same. Thus, diffusion layers 18 used as source anddrain regions are formed to configure a structure as shown in FIG. 18I.

The succeeding process is not shown in the drawing here, but like thethird embodiment, for example, an inter-level insulating film isdeposited by the CVD method, then contact holes are formed, electrodesformed of aluminum films are formed and then the heat treatment isperformed in a nitrogen atmosphere.

Thus, according to the present embodiment, since the inter-polyinsulating film between the floating gate electrode 14 and the controlgate electrode 16 can be formed of the lanthanum aluminate film 15 bwhich is a ferroelectric film, the coupling ratio between the floatinggate electrode 14 and the control gate electrode 16 can be increased.Further, since the tunnel insulating film 13 can be made thin, the writevoltage can be further lowered. Further, it is not necessary to enlargethe area of the insulating film 15 b between the floating gate electrode14 and the control gate electrode 16 in order to enhance the couplingratio, the area of the insulating film 15 b can be set equal to that ofthe tunnel insulating film 13 and the gate portion can be formed as asimple flat laminated structure. As a result, a problem of interferencebetween cells caused by miniaturization of the device can be solved.

Further, if a High-k film such as a metal oxide film is used between thefloating gate electrode 14 and the control gate electrode 16, metalimpurities are diffused via a grain boundary of the polysilicon film ofthe floating gate electrode 14 and introduced into the tunnel insulatingfilm 13. In the conventional tunnel insulating film, since metalimpurities are easily introduced into the film, a phenomenon that trapsare formed and the withstand voltage is markedly lowered occurs. On theother hand, the silicon nitride film 13 a which is stable in structurecan suppress diffusion of metal impurities by using the tunnelinsulating film 13 of the present embodiment. Further, because of thelaminated structure of the silicon oxide films 13 b, 13 c and siliconnitride film 13 a, defects caused by metal impurities formed at the timeof stress application can be suppressed from being connected from thefloating gate electrode 14 to the silicon substrate interface.Therefore, deterioration in the dielectric strength can be suppressed.

In the above embodiment, a case wherein the lanthanum aluminate film(LaAlOx) is formed as the ferroelectric film 15 b lying between thefloating gate electrode 14 and the control gate electrode 16 isexplained, but this invention is not limited to this case. For example,the same effect can be attained by using metal oxide such as aluminiumoxide (Al₂O₃) or hafnium oxide (HfO₂) having a high dielectric constant,aluminate film (MAlOx, M: metal element, x: percentage of oxygen) orsilicate film (MSiOx, M: metal element, x: percentage of oxygen).Further, the same effect can be attained by using oxide of transitionmetal such as HfO2, ZrO2, CeO2 or La2O3, LaSiOx or the like. If theferroelectric film between the electrodes 14 and 16 is formed of alaminated structure of aluminium oxide and hafnium oxide, defects suchas oxygen loss in the aluminium oxide film can be restored by use ofatomic oxygen at low temperatures and thus a structure in which the leakcurrent can be suppressed to a low level can be realized.

Fifth Embodiment

FIGS. 19A to 19E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to afifth embodiment of this invention. Portions which are the same as thoseof FIG. 1 are denoted by the same reference symbols and the detailexplanation thereof is omitted.

FIG. 19A shows a state in which a silicon oxide film 13 b is formed on asilicon substrate 11 and a silicon nitride film 13 a is formed thereonas shown in FIG. 12C. In the step shown in FIG. 12D, for example, thesilicon oxide film is formed to sandwich the silicon nitride filmtogether with the above silicon oxide film by exposing the resultantstructure to oxidizing gas such as oxygen after formation of the siliconnitride film. Since the silicon nitride film has 3-coordinate bonds, theoxidation speed of the surface thereof is low.

In order to solve the above problem, in the present embodiment, as shownin FIG. 19B, a silicon film 23 is deposited on the silicon nitride film13 a after formation of the silicon nitride film 13 a. Then, as shown inFIG. 19C, the silicon film 23 is exposed to oxidizing gas 20 such asoxygen, for example. As a result, as shown in FIG. 19D, the siliconoxide film 23 is oxidized and converted into a silicon oxide film 13 c.Further, an oxidizing agent passes through the silicon nitride film 13 ato increase the film thickness of the silicon oxide film 13 b in an areabetween the substrate 11 and the silicon nitride film 13 a.

Next, as shown in FIG. 19E, a polysilicon film is deposited on thesilicon oxide film 13 c to form a floating gate electrode 14.

In the process of the present embodiment, the film thickness of thesilicon oxide film 13 c on the silicon nitride film 13 a can becontrolled according to the film thickness of the deposited silicon film23. In this case, as the silicon film 23 formed on the silicon nitridefilm 13 a, an amorphous silicon film, polysilicon film or single crystalsilicon film can be used, but it is preferable to use amorphous siliconin order to uniformly deposit the silicon film on the surface of thesilicon nitride film.

There occurs no problem even if a polysilicon film is used as thesilicon film 23. However, since it has crystal grains having differentplane orientations, silicon grains will remain on the silicon oxide film13 c in some cases when an amount of oxygen is insufficient. In thiscase, as shown in FIG. 20, it is possible to form a semiconductor memorydevice based on the quantum effect by accumulating charges on thesilicon grains 24. If the silicon film is deposited by using, forexample, silane (SiH₄) gas or dichlorosilane (SiH₂Cl₂) gas at a hightemperature of 700° C., the deposited silicon film becomes a singlecrystal silicon film. Since the oxidation process uniformly proceeds ifthe single crystal silicon film is used, the silicon oxide film 13 cwith higher quality can be formed.

The “single crystal film” used here indicates a film having uniformcrystallinity in the C-axis direction (vertical direction with respectto the interface) and having uniform crystallinities in the a-axis andb-axis directions (in the plane).

Sixth Embodiment

FIG. 21 is a cross sectional view showing the schematic structure of anonvolatile semiconductor memory device according to a sixth embodimentof this invention. Portions which are the same as those of FIG. 1 aredenoted by the same reference symbols and the detail explanation thereofis omitted.

In the present embodiment, a silicon thermal oxide film (elementisolation insulating film) 12 for element isolation is embedded in andformed on a p-type silicon substrate 11. On the surface of the siliconsubstrate 11, n-type source and drain diffusion layers 18 are formed byion-implantation of phosphorus. Further, as shown in FIG. 2, on thesurface of the silicon substrate 11, a tunnel insulating film (firstgate insulating film) 13 having the laminated structure of a siliconnitride film 13 a sandwiched between silicon oxide films 13 b, and 13 cis formed.

On the tunnel insulating film 13, a nickel silicide film 14 a used as afloating gate electrode is formed. Further, a lanthanum aluminate film15 b with a thickness of 15 nm is formed as a second gate insulatingfilm on the floating gate electrode 14 a. Then, a polysilicon film 16used as a control gate electrode is formed on the second gate insulatingfilm 15 b. Further, a silicon oxide film 17 is formed on the side wallsof the gate portion.

In the nonvolatile semiconductor memory with the thus configuredstructure, the width of the depletion layer extending in the electrodeby application of voltage can be suppressed in comparison with theconventional floating gate electrode formed of a polysilicon film.Therefore, an electric field can be effectively applied to the tunnelinsulating film 13 and the write voltage can be lowered.

FIG. 22 is a characteristic diagram showing the above effect. When thesilicon nitride film having a dielectric constant larger than that ofthe silicon oxide film is used, the width of the depletion layer in thepolysilicon film becomes larger in comparison with a case of theconventional silicon oxide film to weaken the electric filed effectivelyapplied to the tunnel insulating film. In order to suppress the aboveeffect, the nickel silicide film 14 a is used as a floating gateelectrode.

Further, the material of the floating gate electrode is not limited tonickel silicide and other silicide such as tungsten silicide can beused. The same effect can be attained if a metal compound such astantalum nitride (TaN) or titanium nitride (TiN) is used. Further, ametal material such as ruthenium (Ru) or tungsten (W) can be used. Inthese cases, since an interface layer of the silicon oxide film whichwill be formed by reaction with the second insulating film lying betweenthe control gate electrode and the floating gate electrode such as apolysilicon film is difficult to be formed, it is effective to make thesecond insulating film thin.

Seventh Embodiment

FIG. 23 is a perspective view showing the schematic structure of afin-type nonvolatile semiconductor memory device according to a seventhembodiment of this invention.

As shown in FIG. 23, the fin-type nonvolatile semiconductor memory is anonvolatile semiconductor memory having transistors configured on aplate-like element region 31 called a fin and has the effects ofminiaturization, high integration density and suppression of theshort-channel effect. Further, with the above fin structure, thelaminated structure having the silicon nitride film sandwiched betweenthe silicon oxide films configures a tunnel insulating film 33 and thesilicon nitride film is formed of a silicon nitride film having3-coordinate nitrogen bonds aggregated at high density to attain thesame effect as that in the first embodiment.

FIGS. 24A to 24H show the manufacturing process of this embodiment. Thedrawing corresponds to the cross sectional view taken along the A-A′line in FIG. 23.

First, as shown in FIG. 24A, a plate-like element region 31 verticallyerected and formed on the silicon substrate 30 is oxidized by use ofoxygen gas, for example, to form a silicon oxide film 33 b with athickness of approximately 2 nm as shown in FIG. 24B. Then, as shown inFIG. 24C, a flat amorphous silicon film 33 d with a thickness ofapproximately 1 nm is formed by use of disilane gas, for example.

Next, as shown in FIG. 24D, the amorphous silicon film 33 d is directlynitrided at a temperature of 700° C. under pressure of 50 Torr by use ofa plasma nitriding method, for example, to form a silicon nitride film33 a with a thickness of approximately 2 nm. At this time, for example,nitrogen is used as the gas source for plasma nitriding, but if thenitriding process is performed by use of a mixture of nitrogen andoxygen, a silicon nitride film (a so-called oxynitride film) havingoxygen introduced therein can be formed. Further, the oxygenconcentration in the silicon nitride film can be controlled to a desiredconcentration by changing the mixing ratio of the nitriding gas tooxidizing gas.

Then, for example, as shown in FIG. 24E, a silicon oxide film 33 c witha thickness of approximately 2 nm is formed by performing the oxidizingprocess by use of oxygen gas, for example. As a result, a tunnelinsulating film 33 formed of a silicon nitride film 33 a containingoxygen and sandwiched between the silicon oxide films 33 b and 33 c isformed. After this, as shown in FIG. 24F, an n-type polysilicon filmwith a thickness of 200 nm having phosphorus doped therein is depositedas a floating gate electrode 34 on the tunnel insulating film 33.

Next, as shown in FIG. 24G, an ONO film 35 with a thickness of 7 nm, forexample, is formed on the polysilicon film 34. Then, as shown in FIG.24H, an n-type polysilicon film with a thickness of 200 nm havingphosphorus doped therein is deposited as a control gate electrode 36 onthe ONO film 35.

The succeeding process is not shown in the drawing here, but like theother embodiments, the patterning process is performed by use of aresist pattern and then the polysilicon films 36, 34, tunnel insulatingfilm 33 and ONO film 35 are etched by use of the reactive ion etchingmethod to form a gate portion. Further, after the resist mask isremoved, the heat treatment is performed in the oxidation atmosphere torestore the condition damaged by the processing and a post-oxidationfilm with approximately 3 nm is formed. Then, for example, afterphosphorus is ion-implanted into the entire surface with theconcentration of 3×10¹⁵ cm⁻², the heat treatment is performed at 1000°C. for 20 seconds, for example, to diffuse phosphorus into the siliconsubstrate and activate the same. Thus, diffusion layers used as sourceand drain regions are formed.

Next, for example, a silicon oxide film with a thickness of 300 nm isdeposited on the entire surface by the CVD method and then contact holesare formed in the silicon oxide film by an anisotropic etching process.After this, an aluminum film with a thickness of 800 nm containingsilicon and copper of 0.5%, for example, is formed and then it ispatterned to form electrodes. Next, the heat treatment is performed at450° C. for 15 minutes in a nitrogen atmosphere containing hydrogen of10%.

Thus, according to the present embodiment, a fin-type nonvolatile memoryhaving the tunnel insulating film of high quality and high reliabilityand attaining the effects of miniaturization, high integration densityand suppression of the short-channel effect can be formed. Further, thetunnel insulating film 33 is formed as a tunnel insulating film whichsuppresses a leak current, and at the same time, the physical filmthickness of the tunnel insulating film is set to 6 nm by setting thethickness of the silicon nitride film sandwiched between the siliconoxide films of 2 nm to 2 nm. Thus, the tunnel insulating film itself canbe made thin. As a result, the power supply voltage can be lowered andit becomes possible to enhance the characteristic of the device andenhance the reliability thereof.

Eighth Embodiment

FIGS. 25A to 25E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to aneighth embodiment of this invention, and more particularly, showing themanufacturing process of a tunnel insulating film. Portions which arethe same as those of FIG. 1 are denoted by the same reference symbolsand the detail explanation thereof is omitted.

The present embodiment is different from the second embodiment explainedbefore in that a crystalline silicon film is formed instead of formationof the amorphous silicon film used as a basis of formation of thenitride film. In this case, crystalline silicon means that the wholeportion of silicon is not in the amorphous state but in the singlecrystal silicon state, polysilicon state or a state in which crystalgrains are combined.

First, as shown in FIG. 25A, a silicon oxide film 13 b with a thicknessof approximately 2 nm is formed by oxidizing the surface of a siliconsubstrate 11 by use of oxygen gas, for example. Then, as shown in FIG.25B, a flat silicon film 13 e with a thickness of approximately 1 nm isformed on the silicon oxide film 13 b by use of silane gas, for example.The condition at this time is that the temperature is 720° C., thepressure is 275 Torr and film formation time is 0.5 second.

Next, as shown in FIG. 25C, the silicon film 13 e is directly nitridedat a temperature of 100° C. under pressure of 0.03 Torr by use of aplasma nitriding method, for example. Thus, a silicon nitride film 13 awith a thickness of approximately 1 nm is formed. At this time, forexample, nitrogen is used as a gas source for plasma nitriding, but ifthe nitriding process is performed by use of a mixture of nitrogen andoxygen, a silicon nitride film (a so-called oxynitride film) havingoxygen introduced therein can be formed. Further, the oxygenconcentration in the silicon nitride film can be controlled to a desiredconcentration by changing the mixing ratio of the nitriding gas tooxidizing gas.

Then, for example, a silicon oxide film 13 c with a thickness ofapproximately 2 nm is formed by oxidizing the surface of the siliconnitride film 13 a by use of oxygen gas (oxidizing gas) 20. As a result,as shown in FIG. 25D, a tunnel insulating film 13 formed of the siliconnitride film 13 a sandwiched between the silicon oxide films 13 b and 13c is formed. After this, as shown in FIG. 25E, an n-type polysiliconfilm with a thickness of 200 nm having phosphorus doped therein isdeposited as a floating gate electrode 14 on the tunnel insulating film13. Thus, a MOS structure can be formed.

In order to form a silicon nitride film having 3-coordinate nitrogenbonds, it is necessary that the silicon film formed in the step shown inFIG. 25B is a flat and continuous film. If the silicon film is nitridedwhile it is not formed as a continuous film and silicon islands (cores)are disorderly provided together, a thin region is locally provided andSi atoms which are not nitrided will remain in the nitride film. This isnot preferable from the viewpoint of an insulating property. Therefore,as will be described later (as will be explained in detail in a ninthembodiment), the deposition condition of a crystalline silicon filmbecomes important.

In addition, it is not preferable that the film is set in thepolycrystalline state. When Si atoms are set in the polycrystallinestate, the nitriding rate of Si in a grain boundary is high, but thenitriding rate of crystal Si in the grain is low. Therefore, thereoccurs a strong possibility that a crystal Si region which is notnitrided will remain. This means that Si dots remain in the film and itis not preferable from the viewpoint of an insulating property.Therefore, as will be described later (as will be explained in detail ina ninth embodiment), estimation of time required for fully nitridingpolycrystalline Si becomes important.

The silicon film can be deposited in a further flat form by increasingthe number of dangling bonds of the surface on which the silicon film isdeposited. That is, if a step of increasing the number of surfacedangling bonds immediately before deposition of silicon is additionallyprovided in the course of depositing silicon, an insulating film whichis excellent in the degree of flatness and insulating property can beformed. The above effect will be explained in detail in the ninthembodiment which will be described later.

In the present embodiment, the silicon film similar to a single crystalfilm can be formed as a continuous film in a flat form by depositingsilicon for 0.5 second or more in a condition of the temperature of 720°C. and pressure of 275 Torr. Then, by nitriding the flat silicon film inthe above condition, the silicon nitride film having the 3-coordinatenitrogen bonds at high concentration can be formed like the case of thesecond embodiment. Therefore, in the present embodiment, the same effectas that of the second embodiment can be attained.

Ninth Embodiment

FIGS. 26A to 26E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to aninth embodiment of this invention, and more particularly, showing themanufacturing process of a tunnel insulating film. Portions which arethe same as those of FIG. 1 are denoted by the same reference symbolsand the detail explanation thereof is omitted.

The present embodiment is different from the eighth embodiment explainedbefore in that the process of depositing and nitriding a crystallinesilicon film is repeatedly performed to form a silicon nitride film withsufficiently large film thickness.

First, as shown in FIG. 26A, a silicon oxide film 13 b with a thicknessof approximately 2 nm is formed by oxidizing the surface of a siliconsubstrate 11 by use of oxygen gas, for example. Then, a silicon film 13e is formed on the silicon oxide film 13 b by use of silane gas, forexample. At this time, a chamber in which both of the film formationprocess by CVD and the nitriding process can be performed is used andthe substrate 11 is disposed inside the chamber to form the silicon film13 e. Specifically, SiH₄ is used as material gas, the gas pressure inthe chamber is set at 275 Torr, the substrate temperature is set at 720°C. and film formation time is set to 1 second. As a result, the siliconfilm 13 e with a thickness of 1 nm is deposited on the surface of thesilicon oxide film 13 b.

Next, NH₃ at 3 Torr is used as the atmosphere in the chamber, thesurface temperature of the silicon film 13 e is set to 700° C. bycontrolling the heater and this state is maintained for 200 seconds.Then, as shown in FIG. 26B, a silicon nitride film 13 a with a thicknessof 1.3 nm is formed.

Next, as shown in FIG. 26C, silane gas is used again to form a siliconfilm 13 e on the silicon nitride film 13 a. At this time, SiH₄ is usedas material gas, the gas pressure in the chamber is set at 275 Torr, thesubstrate temperature is set at 720° C. and film formation time is setto 0.5 second. As a result, a silicon film 13 e with a thickness of 1 nmis deposited on the surface of the silicon oxide film 13 b.

Next, as shown in FIG. 26D, the silicon film 13 e is converted into anitride film by performing the nitriding process at the temperature of700° C. for 200 seconds like the above case and the film thickness ofthe silicon nitride film 13 a is increased by 1.2 nm. At this time, asilicon nitride film with approximately 6 nm can be formed by repeatedlyperforming the steps of FIGS. 26C and 26D by five times, for example.That is, by repeatedly performing the steps shown in FIGS. 26C and 26Dby plural times, a silicon nitride film having 3-coordinate nitrogenbonds at high density can be formed to necessary film thickness.

In the present embodiment, the silicon film forming process andnitriding process are performed in the same chamber, but they can beperformed in independent chambers. In this case, the film formationchamber and nitriding chamber are arranged in adjacent positions and thetwo chambers are connected in an airtight fashion.

Next, like the former embodiment, as shown in FIG. 26E, for example, thesurface of the silicon nitride film 13 a is oxidized by use of oxygengas (oxidizing agent) 20 to form a silicon oxide film 13 c with athickness of approximately 2 nm. Thus, a tunnel insulating film 13formed of the silicon nitride film 13 a sandwiched between the siliconoxide films 13 b and 13 c is formed. After this, like the formerembodiment, a MOS structure can be formed by forming a floating gateelectrode 14 on the tunnel insulating film 13.

It should be noted in this embodiment that the silicon film is formed ina flat and continuous form in the steps shown in FIGS. 26A and 26C. Ifthe nitriding process is performed while the silicon film is not formedas a continuous film and silicon islands (cores) are disorderly providedtogether and the depositing and nitriding steps are repeatedlyperformed, then a film with irregular thickness is formed. This causes athin region to be locally provided and it is not preferable from theviewpoint of an insulating property.

In addition, the polycrystalline state is not preferable. When Si atomsare set in the polycrystalline state, the nitriding rate of Si in agrain boundary is high, but the nitriding rate of crystal Si in thegrain is low. Therefore, there occurs a strong possibility that acrystal Si region which is not nitrided will remain. This is notpreferable from the viewpoint of an insulating property. Therefore, aswill be described later, estimation of time required for completelynitriding polycrystalline Si becomes important.

FIG. 27 shows a variation in the film thickness of a nitride film formedwhen a silicon film is formed on the silicon substrate by changingdeposition time and then subjected to a nitriding process at 700° C. for1000 seconds. It is understood that the film thickness starts toincrease when the deposition time exceeds approximately 0.5 second andthe film thickness rapidly increases when the deposition time exceedsapproximately 5 seconds. This is based on the following reason.

The reason why an increase in the film thickness of the nitride filmcannot be observed in the deposition time of less than 0.5 second isthat silicon is not deposited or a continuous silicon film is not formedin the deposition time of less than 0.5 second. The reason why the filmthickness of the nitride film starts to increase when the depositiontime exceeds approximately 0.5 second is that silicon is deposited toform a continuous film and is set not in the polycrystalline state butin the single crystal state (in the same state as bare silicon) when thedeposition time exceeds approximately 0.5 second. Further, the reasonwhy the film thickness rapidly increases when the deposition timeexceeds approximately 5 seconds is that polysilicon is deposited onsingle crystal silicon when the deposition time exceeds approximately 5seconds. That is, since the nitriding rate of polysilicon is high, thefilm thickness of the nitride film rapidly increases when the depositiontime exceeds approximately 5 seconds. The tendency is the same when thebackground is formed of a silicon oxide film or silicon nitride film.

Therefore, whether or not silicon is deposited to form a continuous filmor whether or not polysilicon is deposited can be checked by observingthe above tendency. That is, in this case, silicon may be deposited inthe time range of 0.5 second to 5 seconds. Likewise, when a silicon filmis deposited on a nitride film, silicon may be deposited in the timerange of 0.5 second to 5 seconds. Further, when a silicon film isdeposited on an oxide film, silicon may be deposited in the time rangeof 5 seconds to 7 seconds.

It should be further noted in the present embodiment that a polysiliconfilm is completely nitrided even when the polysilicon film is formed inthe steps shown in FIGS. 26B and 26D. By completely nitriding thepolysilicon film, a network of silicon and nitrogen is formed, thedensity of the 3-coordinate nitrogen bonds can be enhanced and a highlyreliable nitride film having less defects can be formed.

In a process of nitriding polysilicon on the silicon substrate, thenitriding rate is first set high and becomes low at a certain time. Thatis, “a region in which the nitriding rate is high” and “a region inwhich the nitriding rate is low” are provided in the relation betweenthe nitriding time and the film thickness to be formed. The “region inwhich the nitriding rate is high” is “a region in which nitrization ofpolysilicon having a high nitriding rate occurs” and the “region inwhich the nitriding rate is low” is “a region in which polysilicon iscompletely nitrided and nitrization of single crystal silicon having alow nitriding rate occurs”. Therefore, time required for completelynitriding polysilicon can be acquired by observing the above relation.

FIG. 28 shows a difference in the SILC characteristic depending on anitride film formation condition. As shown in the present embodiment, itis understood that a leak current in a low-voltage region is morerapidly reduced in a film formed by plasma-nitriding the siliconsubstrate in a condition of 700° C. and 3 Torr and having less defectsthan in a film formed by plasma-nitriding the silicon substrate at roomtemperature and having more defects. This is because occurrence ofdefects is suppressed by forming a network of Si and N and the number ofleak paths in the bulk is reduced.

FIG. 29 shows the nitride film thickness attained in a case where theprocess of nitriding the silicon substrate for 200 seconds is repeatedlyperformed five times (normal nitriding) without depositing silicon afterthe silicon substrate is nitrided at 700° C. for 100 seconds incomparison with the nitride film thickness attained in a case where theprocess of nitriding the silicon substrate for 200 seconds is repeatedlyperformed five times (cycle nitriding) after silicon is deposited forone second. It is understood that the physical film thickness can bemade larger by 4 nm or more by depositing silicon even if the filmformation time is set to the same. The difference is caused by making itmore difficult for nitrogen to pass through the nitride film as thenitride film becomes thicker. By performing the process of depositingand nitriding silicon on the nitride film, nitrogen can always reactwith silicon even if it does not diffuse into the nitride film. This isthe reason why the film thickness can be made large even when the samefilm formation time is used.

The nitriding time is set to 200 seconds, but it can be set longer. Theimportant thing is that silicon deposited on the nitride film iscompletely nitrided. FIG. 30 shows the characteristic of a nitride filmwhen silicon is deposited for one second and nitrided for 100 secondsafter the silicon substrate is nitrided at 700° C. for 100 seconds incomparison with the characteristic of a nitride film when silicon isnitrided for 200 seconds. In the case of nitriding for 100 seconds, itis understood that a CV curve is shifted to the negative side. This isbecause a large amount of defects are present in the film since siliconis not completely nitrided when the nitriding time is not sufficientlylong.

The silicon deposition time is set to one second, but it can be madelonger. However, time required for fully nitriding silicon is requiredto be made longer accordingly. FIG. 31 shows deposition time of silicon(single crystal silicon and polysilicon) and time required forcompletely nitriding the same. It is understood that time required forcompletely nitriding silicon becomes markedly long as the depositionfilm thickness is made larger. This is because nitrogen must diffusethrough a thick nitride film to completely nitride silicon when thesilicon film becomes thick even if silicon is set in a polycrystallinestate. Therefore, in order to form a thick film, it is efficient torepeatedly perform a process of depositing a thin polysilicon film andquickly and fully nitriding the same.

In this case, deposition and nitriding of crystalline silicon can berapidly switched by performing film formation and nitriding ofcrystalline silicon in the same chamber as in the present embodiment.One deposition step of crystalline silicon can be performed in anextremely short period of time, for example, several seconds. Therefore,unlike the normal nitriding, even if the number of switching operationsof film formation and nitriding of crystalline silicon is increased andtime required for the switching operation is additionally taken, thetotal processing time can be made shorter in the cycle nitridingprocess.

Further, silicon deposition time should not be set excessively short.For silicon deposition, incubation time becomes important. That is, ifdeposition time is shorter than the incubation time, it indicates thatsilicon is not deposited or silicon does not form a continuous film. Inthis case, “formation of the continuous film” indicates “a state inwhich the background (silicon oxide film) having a film depositedthereon is not exposed”. FIG. 32 shows the ratio of incubation times ofsilicon on the oxide film, Si and nitride film. It is understood thatthe incubation time becomes shorter in an order of oxide film→Si→nitridefilm. The reason why the incubation time is different depending on thesurfaces of respective types of films is that the number of danglingbonds present in the surface thereof is different.

The film thickness of silicon which forms a continuous film is differentdepending on the number of dangling bonds (bonds which are notconnected) of the surface on which silicon is deposited. Time until thecontinuous film is formed becomes shorter in a case where a silicon filmis formed on a nitride film (Si₃N₄) than on Si. This is because thenumber of dangling bonds of the surface of the nitride film is largerthan that on the Si surface. Time until the continuous film is formedbecomes longer in a case where a film is formed on an oxide film (SiO₂)than on Si. This is because the number of dangling bonds of the surfaceof the oxide film is smaller than that of the Si surface.

When the dangling bonds of the surface are less, a film is grown throughthe growing process called a Volmer-Weber type. In the Volmer-Webertype, a plurality of atoms are condensed to form cores on the substrateand flying atoms are sequentially concentrated to grow the cores in a3-dimensional form. The cores (islands) grow and are combined as thevaporization process is performed and they form a continuous film. Whenthe interaction between the vaporization atoms is stronger than thatbetween the vaporization atom and the substrate atom, the above growingprocess is performed.

When the number of dangling bonds is large, a film is grown through agrowing process called a Stranski-Krastanov type (a film is grown in asingle-layered (or plural-layered) form on the substrate and then3-dimensional cores are formed and grown thereon). That is, when a largenumber of dangling bonds are present on the surface, a continuous filmcan be easily formed from the beginning. Therefore, time required fordepositing silicon to form the continuous film is different depending onthe type of the surface.

Based on the relation shown in FIG. 32, time required for deposition isdifferent in a case where the silicon film 13 e is deposited in the stepshown in FIG. 26A and in a case where the silicon film 13 e is depositedin the step shown in FIG. 26C. In this embodiment, approximately 5seconds is set when the silicon film 13 e is deposited on the siliconoxide film 13 b and approximately 0.5 second is set when the siliconfilm 13 e is deposited on the silicon nitride film 13 a.

Within the time period shorter than the incubation time, silicon is notdeposited or a continuous film is not formed. FIG. 33 shows thecharacteristic obtained when silicon is repeatedly deposited five timesto increase the film thickness. In each case, the film thickness is setto 3 nm. It is understood that the insulating property of a nitride filmformed by repeatedly performing a process of depositing silicon for 0.1second in a discontinuous film state and nitriding silicon to increasethe film thickness is markedly lowered in comparison with that of anitride film formed by repeatedly performing a process of depositingsilicon for 0.5 second to form a continuous film and nitriding silicon.This is because a region with small film thickness is provided since thefilm is made thick without forming a continuous film.

That is, silicon can be deposited in a flat form by increasing thenumber of dangling bonds of the surface on which silicon is deposited.FIG. 34 shows a variation in the surface roughness when silicon isdeposited on the silicon oxide film and when silicon is deposited afterthe surface of the silicon oxide film is plasma-nitrided for 5 secondsby use of a power of 100 W under 0.08 Torr in a He atmosphere which isdiluted to 40% by use of N₂ (after dangling bonds are formed on thesurface). It is understood that the degree of surface roughness ofsilicon becomes low by nitriding the surface of the silicon oxide film.This is because the number of dangling bonds of the surface of thesilicon oxide film is increased by nitriding and the growing process ofsilicon is changed from the Volmer-Weber type to the Stranski-Krastanovtype. Therefore, if a step of increasing the number of surface danglingbonds immediately before depositing silicon is added in the course ofdepositing silicon, an insulating film having excellent flatness andinsulating property can be formed. As one example, the plasma-nitridingprocess is provided.

As described above, a silicon nitride film having 3-coordinate nitrogenbonds at high density can be made thick by repeatedly performing theprocess of depositing and nitriding silicon. Thus, a thick nitride filmcan be formed in a short period of time by setting adequate silicondeposition time and nitriding time.

In the present embodiment, the film formation temperature of crystallinesilicon is set at 720° C., silane is used as the material gas and thegas pressure is set at 275 Torr, but the above condition can beadequately changed. For example, the temperature is preferably set inthe range of 550° C. to 1050° C. as the temperature condition.

NH₃ is used as nitrogen gas used at the time of the nitriding process,but the nitriding process can be performed by using another gascontaining nitrogen. The nitriding temperature is set at 700° C., butthe nitriding process can be performed by using other temperatures.Further, the nitriding pressure is set at 3 Torr, but the nitridingprocess can be performed at different pressure. For example, thetemperature is preferably set in the range of 600° C. to 850° C. as thetemperature condition at the time of the nitriding process. Further, thegas pressure is preferably set at 100 Torr or less.

The number by which the step of depositing crystalline silicon on thenitride film and nitriding the same is not limited to five times and canbe set to any number of times. Further, after polysilicon is depositedon the nitride film, the silicon film can be oxidized by use of gascontaining oxygen.

Tenth Embodiment

FIGS. 35A to 35E are cross sectional views showing the manufacturingprocess of a nonvolatile semiconductor memory device according to atenth embodiment of this invention, and more particularly, showing themanufacturing process of a tunnel insulating film. Portions which arethe same as those of FIG. 1 are denoted by the same reference symbolsand the detail explanation thereof is omitted.

The present embodiment is different from the ninth embodiment explainedbefore in that amorphous silicon is used instead of crystalline siliconand the other basic process is the same.

First, as shown in FIG. 35A, the surface of a silicon substrate 11 isoxidized by use of oxygen gas, for example, to form a silicon oxide film13 b with a thickness of approximately 2 nm. Then, an amorphous siliconfilm 13 d is formed on the silicon oxide film 13 b by use of silane gas,for example. In this example, a chamber in which both of the filmformation process by CVD and the nitriding process could be performedwas used and the amorphous silicon film 13 d was formed with thesubstrate 11 arranged in the chamber. Specifically, Si₂H₆ was used asmaterial gas, the gas pressure in the chamber was set at 1 Torr, thesubstrate temperature was set at 400° C. and the film formation time wasset to 40 minutes. As a result, the amorphous silicon film 13 d with athickness of 2 nm was deposited on the surface of the silicon oxide film13 b.

Next, NH₃ of 3 Torr, for example, is used as the atmosphere in thechamber, a heater is controlled to set the surface temperature of theamorphous silicon film 13 d to 700° C. and this state is maintained for300 seconds or longer. Thus, as shown in FIG. 35B, a silicon nitridefilm 13 a with a thickness of 2.2 nm is formed.

Next, as shown in FIG. 35C, an amorphous silicon film 13 d is formed onthe silicon nitride film 13 a by use of silane gas again. In thisexample, Si₂H₆ was used as material gas, the gas pressure in the chamberwas set at 1 Torr, the substrate temperature was set at 400° C. and thefilm formation time was set to 40 minutes. As a result, the amorphoussilicon film 13 d with a thickness of 2 nm was deposited on the surfaceof the silicon oxide film 13 b.

After this, as shown in FIG. 35D, like the above case, the amorphoussilicon 13 d is converted into a nitride film by performing thenitriding process at 700° C. for 300 seconds and the film thickness ofthe silicon nitride film 13 a is increased by 2.2 nm. At this time, asilicon nitride film with required film thickness can be obtained byrepeatedly performing the steps shown in FIGS. 35C and 35D.

In the present embodiment, the amorphous silicon film formation processand nitriding process are performed in the same chamber, but theprocesses may be performed in independent chambers. In this case, thefilm formation chamber and nitriding chamber are arranged adjacent toeach other and both of the chambers may be connected in an airtightfashion.

Next, like the former embodiment, as shown in FIG. 35E, the surface ofthe silicon nitride film 13 a is oxidized by use of, for example, oxygengas (oxidizing agent) 20 to form a silicon oxide film 13 c with athickness of approximately 2 nm. Thus, a tunnel insulating film 13formed of the silicon nitride film 13 a sandwiched between the siliconoxide films 13 b and 13 c is formed. After this, like the formerembodiment, a MOS structure can be formed by forming a floating gateelectrode 14 on the tunnel insulating film 13.

Also, in the present embodiment, like the ninth embodiment, it isimportant to make the amorphous silicon film in a flat and continuousform in the steps shown in FIGS. 35A and 35C. Further, it is importantto completely nitride the amorphous silicon film in the steps shown inFIGS. 35B and 35D. The condition to satisfy the above requirements maybe set to substantially the same condition as that of the ninthembodiment, but it is necessary to set the temperature at the time offormation of the amorphous silicon film lower than that in the ninthembodiment so as to prevent occurrence of crystallization. For example,it is preferable to set the temperature condition in a range of 300° C.to 550° C.

Also, in the present embodiment, the tunnel insulating film 13 havingthe structure in which the silicon nitride film is sandwiched betweenthe silicon oxide films 13 b and 13 c can be formed and the siliconnitride film 13 a having 3-coordinate nitrogen bonds at high density canbe formed with sufficiently large thickness. Therefore, the same effectas that of the ninth embodiment can be obtained.

Eleventh Embodiment

FIGS. 36 and 37 illustrate a CMOS semiconductor device according to aneleventh embodiment of this invention, FIG. 36 is an equivalent circuitdiagram and FIG. 37 is a cross sectional view of the element structure.In this case, a CMOS inverter with the simplest configuration isexplained as an example.

As shown in FIG. 36, the CMOS inverter has a configuration in which ap-type MOSFET and an n-type MOSFET are serially connected. It is acircuit which outputs 0V when the diffusion layer on one side of then-type MOSFET is grounded, drive voltage is applied to the diffusionlayer on one side of the p-type MOSFET and drive voltage is applied tothe gate electrodes of the p-type and n-type MOSFETs as an input.

In the present embodiment, as shown in FIG. 37, a silicon thermal oxidefilm (element isolation insulating film) 102 for element isolation isembedded in and formed on a p-type silicon substrate 101. A p-type wellregion 103 a is formed on the n-type MOSFET side by ion-implanting boronand an n-type well region 103 b is formed on the n-type MOSFET side byion-implanting phosphorus on the surface of the silicon substrate 101.Well electrodes 107 a, 107 b are respectively formed on the well regions103 a, 103 b. Further, n-type source and drain diffusion layers 106 aare formed on the well region 103 a by ion-implanting phosphorus andp-type source and drain diffusion layers 106 b are formed on the wellregion 103 b by ion-implanting BF₂.

Further, gate insulating films 104 which are each formed of a laminatedlayer structure having a silicon nitride film sandwiched between siliconoxide films are formed on the surface of the silicon substrate 101. Thegate insulating film 104 has a structure shown in FIG. 2, for example.That is, the silicon nitride film sandwiched between the silicon oxidefilms is a film which is continuous in the in-plane direction and thebonding state of nitrogen in the silicon nitride film is a 3-coordinatebond.

On the respective gate insulating films 104, an n-type polysilicon film105 a and p-type polysilicon film 105 b which are used as gateelectrodes are respectively formed on the n-type MOSFET side and p-typeMOSFET side. Further, silicon oxide films 108 are respectively formed onthe surroundings of the gate portions.

In the CMOS semiconductor device thus configured, the gate insulatingfilm 104 has a laminated structure of the silicon oxide film/siliconnitride film/silicon oxide film. Therefore, occurrence of defects orcharge traps caused in the film at time of application of voltage can besuppressed more effectively in comparison with a case wherein theconventional silicon oxide film or oxide-nitride film (oxynitride film)is used. Therefore, enhancement of the long reliability of a circuitcontaining the CMOS inverter can be attained and deterioration in thecharacteristic due to degradation thereof can be suppressed.

FIG. 38 is a diagram showing the characteristic of the CMOS inverteraccording to the present embodiment in comparison with thecharacteristic of the conventional CMOS inverter. In this case, thenegative bias temperature instability of the p-type MOSFET is taken asan example.

The negative bias temperature instability is the degree of reliabilitywith which the threshold voltage of a transistor is shifted when thetransistor is left as it is under high temperatures while negative biasis applied to the p-type gate electrode and an inverted layer is formed.In the case of the CMOS inverter, deterioration that the switchingcharacteristic becomes low occurs. It is known that the nitrogenconcentration of a portion near the interface of the silicon substrateincreases in the conventional method and deterioration becomessignificant, particularly, when the nitrogen concentration becomes high.

According to FIG. 38, it is understood that the service life of a gateinsulating film of high quality according to the present embodiment,that is, a gate insulating film having substantially the same nitrogenconcentration and having a silicon nitride film in which formation ofdefects is suppressed and which is sandwiched between silicon oxidefilms is made longer by two order in comparison with that of theconventional gate insulating film formed of an oxide-nitride filmcontaining nitrogen with high concentration of approximately 30%. Thisis attained by the following reason. That is, in the conventionalstructure, the threshold voltage is shifted by formation of theinterface level by stress application and capture of holes into defectsin the film. On the other hand, in the structure of the presentembodiment, since the silicon oxide film containing no nitrogen isdisposed on the silicon substrate interface and the silicon nitride filmof high quality in which formation of defects is suppressed in the filmis used, formation of the interface level and capture of holes in thefilm can be suppressed.

(Modification)

This invention is not limited to the above embodiments. In theembodiments, the silicon substrate is used, but another semiconductorsubstrate can be used.

Further, in the step of forming the first gate insulating film, thenitriding temperature and pressure can be adequately changed when anamorphous silicon film is nitrided to form a silicon nitride film.However, in order to attain 3-coordinate nitrogen bonds which arecontinuous in the in-plane direction, it is preferable to set thetemperature lower than 800° C. Further, it is desirable to set thepressure lower than 13300 Pa when the temperature is set equal to orhigher than 800° C. When the crystalline silicon film is nitrided toform a silicon nitride film, it is preferable to set the temperature ina range of 500° C. to 850° C. and set the pressure equal to or lowerthan 40 Torr. When a cyclic nitriding process is performed, the numberof repetitive processes can be adequately changed according to acondition such as required nitride film thickness.

When a single-layered film is used as the second gate insulating film,an insulating film having the dielectric constant larger than that ofthe silicon oxide film is preferably used in order to increase thecoupling ratio of the control gate electrode and the floating gateelectrode. For example, it is desirable to use a metal oxide film, metalsilicate film or metal aluminate film. Further, a material of thecontrol gate electrode and floating gate electrode is not limited topolysilicon and it is possible to use another conductive material.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A manufacturing method of a nonvolatile semiconductor memory device,comprising: forming a tunnel insulating film including a three-layeredstructure in which a silicon nitride film is sandwiched between a firstsilicon oxide film and a second silicon oxide film, by: oxidizing asurface of a silicon layer to form the first silicon oxide film,depositing an amorphous silicon film on the first silicon oxide film,nitriding the amorphous silicon film at a temperature not lower than600° C. and not higher than 1000° C. and under a pressure of not lowerthan 13.3 Pa and not higher than 13300 Pa, in order to form the siliconnitride film, and forming the second silicon oxide film on the siliconnitride film.
 2. The method according to claim 1, wherein the amorphoussilicon film is formed at temperatures lower than 550° C. to deposit theamorphous silicon film on the first silicon oxide film.
 3. The methodaccording to claim 1, wherein the surface of the silicon nitride film isoxidized after the silicon nitride film is formed, in order to form asecond silicon oxide film on the silicon nitride film.
 4. The methodaccording to claim 3, wherein the silicon nitride film is subjected toheat treatment before the second silicon oxide film is formed.
 5. Themethod according to claim 4, wherein the heat treatment to which thesilicon nitride film is subjected before the second silicon oxide filmis formed is performed at temperatures not lower than the nitride filmformation temperature.
 6. The method according to claim 1, wherein anamorphous silicon film is formed on the silicon nitride film and thenthe amorphous silicon film is oxidized to form the second silicon oxidefilm on the silicon nitride film.
 7. The method according to claim 1,wherein the depositing and the nitriding of the amorphous silicon filmare performed in a same chamber.
 8. A manufacturing method of anonvolatile semiconductor memory device, comprising: forming a tunnelinsulating film including a three-layered structure in which a siliconnitride film is sandwiched between a first silicon oxide film and asecond silicon oxide film, by oxidizing a surface of a silicon layer toform the first silicon oxide film, depositing an amorphous silicon filmon the first silicon oxide film, nitriding the amorphous silicon film toform the silicon nitride film, and forming the second silicon oxide filmon the silicon nitride film, wherein the depositing and the nitriding ofthe amorphous silicon film are repeatedly performed plural times to formthe silicon nitride film.
 9. The method according to claim 8, whereinthe depositing and the nitriding of the amorphous silicon film areperformed in a same chamber.
 10. The method according to claim 8,wherein the amorphous silicon film is formed at temperatures lower than550° C. to deposit the amorphous silicon film on the first oxide film.11. The method according to claim 8, wherein the surface of the siliconnitride film is oxidized after the silicon nitride film is formed, inorder to form the second silicon oxide film on the silicon nitride film.12. The method according to claim 11, wherein the silicon nitride filmis subjected to heat treatment before the second silicon oxide film isformed.
 13. The method according to claim 12, wherein the heat treatmentto which the silicon nitride film is subjected before the second siliconoxide film is formed is performed at temperatures not lower than thenitride film formation temperature.
 14. The method according to claim 8,wherein an amorphous silicon film is formed on the silicon nitride filmand then the amorphous silicon film is oxidized to form the secondsilicon oxide film on the silicon nitride film.
 15. A manufacturingmethod of a nonvolatile semiconductor memory device, comprising: forminga tunnel insulating film including a three-layered structure in which asilicon nitride film is sandwiched between a first silicon oxide filmand a second silicon oxide film, by oxidizing a surface of a siliconlayer to form the first silicon oxide film, depositing a crystallinesilicon film on the first silicon oxide film, nitriding the crystallinesilicon film to form the silicon nitride film, and forming the secondsilicon oxide film on the silicon nitride film, wherein the depositingand the nitriding of the crystalline silicon film are repeatedlyperformed plural times to form the silicon nitride film.
 16. The methodaccording to claim 15, wherein the crystalline silicon film is formed ina temperature range of 550° C. to 1050° C. to deposit the crystallinesilicon film on the first silicon oxide film.
 17. The method accordingto claim 15, wherein the crystalline silicon film is formed in a rangeof 0.5 second to 5 seconds to deposit the crystalline silicon film onthe first silicon oxide film.
 18. The method according to claim 15,wherein the surface of the first silicon oxide film is plasma-nitrizedafter the first silicon oxide film is formed and before the crystallinesilicon film is deposited on the first silicon nitride film.
 19. Themethod according to claim 15, wherein a deposition process of thecrystalline silicon film is performed for a period of time longer thanincubation time required for making the crystalline silicon filmcontinuous to deposit the crystalline silicon film on the first siliconoxide film.
 20. The method according to claim 15, wherein the depositingand the nitriding of the crystalline silicon film are performed in thesame chamber.
 21. The method according to claim 15, wherein theamorphous silicon film is nitride in a condition such that the nitridingtemperature is set in a range of 600° C. to 1000° C.
 22. The methodaccording to claim 21, wherein the nitriding pressure is set in a rangeof 13.3 Pa and 13300 Pa.
 23. The method according to claim 15, whereinthe crystalline silicon film is nitride in a condition such that thenitriding temperature is set in a range of 500° C. to 850° C.
 24. Themethod according to claim 23, wherein the nitriding pressure is setlower than 532 Pa.